Suspensions often include particles of interests that are difficult to extract and isolate for analysis because the particles occur with such low frequency. For example, blood is a suspension of various particles that is routinely examined for the presence of abnormal organisms or cells, such as circulating tumor cells (“CTCs”), fetal cells or ova, parasites, microorganisms, and inflammatory cells. CTCs are of particular interest because CTCs are cancer cells that have detached from a primary tumor, circulate in the bloodstream, and may be regarded as seeds for subsequent growth of additional tumors (i.e., metastasis) in different tissues. As a result, detecting, enumerating, and characterizing CTCs may provide valuable information in monitoring and treating cancer patients. Although detecting CTCs may help clinicians and cancer researchers predict a patient's chances of survival and/or monitor a patient's response to cancer therapy, CTC numbers are typically very small and are not easily detected. In particular, typical CTCs are found in frequencies on the order of 1-10 CTCs per milliliter sample of whole blood obtained from patients with a metastatic disease. By contrast, a single milliliter sample of whole blood typically contains a few million white blood cells and 4-6 billion red blood cells. In addition to detecting, enumerating, and characterizing CTCs in formulating a therapeutic cancer treatment, it may also be valuable to have additional CTC information such as nucleic acid or proteomic signatures. For example, a CTC distribution and potentially even the CTC itself may vary over time after cancer therapy begins. However, in order to obtain this additional information, the CTCs have to be harvested from a whole blood sample and analyzed with molecular techniques.
Practitioners, researchers, and those working with suspensions continue to seek systems and methods for detecting, enumerating, characterizing, and harvesting various kinds of particles found in a suspension.